The invention relates to methods and apparatus for altering the lateral bending compliance of elongated flexible structures.
2. Power Line Forces on Tools
Movable tools are often driven by power transmitted from a separate source over flexible power lines (e.g., electrical cables, hoses carrying pressurized liquids or gases, or a rotating cable). Tool mountings (and human operators, in the case of hand-held tools) are subject to translational and rotational forces exerted on the tool by the power lines which effectively tether the tool to the power source. For hand tools, variable tether forces combine with the tool weight, inertial forces within the tool, and reaction forces applied to the tool by the workpiece to yield time-varying total linear force and total torque resultant vectors acting on the tool. For hand-held tools, the resultant linear force and torque vectors must be completely offset by counteracting linear forces and torques manually applied by a person wishing to effectively control the tool.
Optimal positioning of a power tool with respect to its tether line(s) can improve controllability by altering the magnitude and/or direction of resultant linear force and torque vectors. When the tether line is particularly stiff and/or heavy (e.g., as a high-pressure air or nitrogen hose) and the range of tool motion is relatively restricted, the tool is often designed with a fixed tether line connection point and spherical tether line-tool angle which minimizes interference of the tether line with tool movement and control. Alternatively, the direction and/or magnitude of tether forces can be adjusted from time-to-time for more general tool movement by changing the spherical angular relationship and/or lateral offset of the tool with respect to a tether line.
Angular measures and offsets are conveniently determined between the tangent to the tether line at the tool connection point and the longitudinal tool axis. In cases where these two lines intersect, the offset is necessarily zero and the spherical tether line-tool angles comprise the longitudinal and circular angles between the tether line and the power tool. The longitudinal tether line-tool angle is that angle between the tether line tangent and the longitudinal tool axis. The circular tether line-tool angle is the angle, measured as projected in a plane substantially perpendicular to the longitudinal tool axis, between a chosen reference or index tool feature (e.g., the tool handle) and the tether line tangent at the connection point. Where the tether line connection point to the tool is laterally offset so that the tangent to the tether line at the connection point does not intersect the longitudinal tool axis, the longitudinal tether line-tool angle is measured about the perpendicular constructed between the axis and the tangent. (Note that the length of this perpendicular is a measure of the lateral offset distance.) The circular tether line-tool angle is measured in a plane perpendicular to the longitudinal tool axis which contains the above constructed perpendicular and a projection of the chosen index tool feature.
A tension force on a laterally offset tether line as defined herein will exert a torque about the longitudinal tool axis (and perhaps one or both of the orthogonal tool axes as well). Even without a lateral offset or tension force, however, a torque may be transmitted directly by the tether line. For example, in many high-pressure hoses, changing gas pressures in the hose (associated with changing tool speeds and loads) result in a reversible longitudinal twisting of the hose which may then be directly transmitted as torque to the tool. This torque may be augmented or cancelled in part by other torques generated during tool movement.
In particular, the degree, direction and speed of change in the spherical tether line-tool angles, the stiffness and weight of the tether line, and any torque or tension (including inertial forces) acting on the tether line will potentially alter the resultant magnitude and direction of any change in tether forces on the tool. Relatively stiff and/or heavy lines will be associated with larger tether force variations during tool movement than lighter and/or more compliant lines. Changes in connection of tether lines to a tool (as, for example, to accommodate left-handed and right-handed operators) will also result in tether force changes that are a function of the line characteristics as well as the connection point location and orientation.
To aid flexibility in the control of tether line forces, commercially available tools (e.g., nitrogen-powered bone drills) may include as optional attachments two or more angular connectors which can be interchangeably mounted on the tool at any of several circular tether line angles while maintaining a substantially constant longitudinal tether line angle. These attachments, e.g., might allow joining a bone drill to a pressurized nitrogen hose at a fixed longitudinal angle of 20.degree., 70.degree., or 90.degree., and at a circular tether line angle between 0.degree. and 180.degree.. The choice of these parameters when the drill is assembled will generally change tether forces on the drill in a predetermined fashion. Due to the wide variety of positions in which a bone drill might be used, however, as well as the substantial range of stiffness in commonly used pressurized nitrogen hoses, the most desired spherical tether line-tool angles (especially the longitudinal angle) may be only approximated with the above attachments. To the extent that actual angles differ from optimal angles, tool controllability may be degraded.
To more closely approximate the specific tether line-tool angles desired, a continuously adjustable compound high-pressure swivel joint may be inserted in the nitrogen line. Such joints, however, are relatively expensive and subject to high-pressure gas leaks. Thus, a need exists for a relatively inexpensive, durable, and easily adjustable means of altering tether line forces on power tools.